SteveF wrote:Have a close look at box 4, you should be able to see a naked quad.

This should allow you to enter a candidate in column 2.

He left out a step of indirection -- the naked quad doesn't yield the candidate, it removes candidates from box four. The changes to the two boxes in column 2 of box four yield a naked trio in column 2 which -- once you've eliminated those from elsewhere in column 2 -- yields the candidate.

The initial quad I found is in cells r4c1, r4c3, r5c3 and r6c3. The only candidates for these four cells are the 4 values of 2, 6, 8 and 9.

As Dusty Chalk correctly points out, this allows you to remove 2, 6, 8, and 9 as candidates from the other cells in box 4.

My next step was to put a 9 in r1c2, it is the only place a 9 can go in column 2, which is the step I think you have found.

I think the triple that others have referred to is in r3c2, r5c2, r6c2? However once you have placed a 9 in r1c2 you don't actually need this, a number of 'only one possibility for a cell' situations lead on from this.